Working machine of these days, for example, a hydraulic excavator is generally provided with an engine, a variable displacement hydraulic pump drivable by power transmitted from the engine, a hydraulic actuator drivable by hydraulic oil delivered from the variable displacement hydraulic pump, an actuator control valve interposed between the variable displacement hydraulic pump and the hydraulic actuator and switchable to one of a feed state in which hydraulic oil is to be fed to the hydraulic actuator and a non-feed state in which the hydraulic oil is returned to a hydraulic oil reservoir without feeding it to the hydraulic actuator, and an exhaust gas purification system for trapping, by a filter, particulate matter in exhaust gas produced by incomplete combustion in the engine.
The exhaust gas purification system is arranged in an exhaust pipe through which the exhaust gas from the engine is guided to an exterior of the hydraulic excavator. The particulate matter trapped in the filter of the exhaust gas purification system is burnt with heat of the exhaust gas, and therefore, is eliminated from the filter.
In consideration of energy savings, the hydraulic excavator is constructed such that in its non-working state, in other words, in the above-mentioned non-feed state of the actuator control valve, an engine output is reduced to a level needed for the variable displacement hydraulic pump to deliver pressure oil at a lowest delivery pressure and a smallest delivery rate required for cooling and lubricating a hydraulic circuit.
When the engine output drops, the temperature of exhaust gas also drops. As a consequence, the burning of particulate matter with the heat of the exhaust gas is rendered difficult to occur, and accordingly, the filter of the exhaust gas purification system becomes prone to clogging. To avoid clogging of the filter, the conventional hydraulic working machine disclosed in Patent Document 1 is constructed such that, when clogging of the filter is detected, the delivery pressure and delivery rate of the variable displacement hydraulic pump are increased to make greater a load to be applied to the engine and the temperature of exhaust gas is allowed to rise to a temperature needed for the burning of particulate matter. A means for increasing the delivery pressure of the variable displacement hydraulic pump is a selector valve that can open or close a line through which delivery oil of the variable displacement hydraulic pump is guided from the variable displacement hydraulic pump to the hydraulic oil reservoir. The hydraulic working machine is constructed such that in the non-feed state of an actuator control valve, the delivery pressure is increased by controlling the selector valve.
In the meanwhile, a construction machine has been proposed in Patent Document 2. This construction machine makes use of an electric motor and an electrical storage device (battery, electric double layer capacitor, or the like) to provide an energy efficiency improved over those of conventional hydraulic working machines such that energy savings can be achieved. In this construction machine disclosed in Patent Document 2, a power generator is connected to an engine, electric energy produced at the power generator during low-load operation is stored in a battery, and upon high-load operation, the electric energy is drawn out to reduce a load on the engine.